Stress fibres-a Ca2+ -independent store for annexins?

Annexins belong to a family of lipid-binding proteins that are implicated in membrane organization. Several members are capable of binding to actin and, in smooth muscle cells, annexin 6 is known to form a Ca(2+)-dependent, plasmalemmal complex with actin filaments. Annexins can also associate with F-actin containing stress fibres within cultured smooth muscle cells or fibroblasts in a Ca(2+)-independent manner. Depolymerization of stress-fibre systems with cytochalasin D leads to the translocation of actin-bound annexin 2 from the cytoplasm to the plasma membrane at high intracellular levels of Ca(2+). This type of Ca(2+)-dependent annexin mobility is observed only in cells of mesenchymal phenotype, which have a well-developed stress-fibre system; not in epithelial cells.